1. Field of the Invention
The present invention relates to a method for molding metallic powder or ceramic powder, and more particularly to a method for molding powders by the use of a thin-wall resilient mold filled up with powders.
2. Description of the Prior Arts
The cold isostatic press method (hereinafter, referred to as "C.I.P. method") is well-known as a method for molding powders wherein a resilient mold made of rubber or the like is filled up with powders such as metallic powder, ceramic powder or the like and sealed, and then an isostatic press is applied to the resilient mold by the use of a pressure medium such as water, oil or the like at ordinary temperatures. However, some idea is required to obtain a compact of desirable shape so that the resilient mold cannot be deformed by weight of powders. Further, a method, wherein a thin-wall resilient mold and a ventilative mold support having an inside shape similar to the shape of the resilient mold are employed, is disclosed in a Japanese Patent Publication Laid Open No. 297402/87. In this method, said thin-wall resilient mold is inserted into said mold support and put close to the inner surface of the mold support by holding the outer surface of the mold support under a negative pressure whereby a shape of the mold is kept. After said mold has been filled up with powder materials, air in the resilient mold has been exhausted and the resilient mold has been sealed, said mold support is removed. After said thin-wall resilient mold holding powder materials therein has been subjected to the cold isostatic press, the thin-wall resilient mold is removed, thereby a compact is prepared.
Initially, problems of the prior art methods for molding powders will be described. In a method of imparting a thickness and strength to the resilient mold, since a degree of contraction of the resilient mold relative to a pressure applied thereto is different from a degree of contraction of a fill-up of powders inside the resilient mold, to which a pressure is applied, the resilient mold and the fill-up do not contract isotropically. Accordingly, the compact is required to be subjected to considerable machining in order to obtain a desired shape and a dimensional accuracy. On the other hand, when a method, wherein the resilient mold with powder is subjected to the C.I.P. treatment, is employed after a thin-wall resilient mold having been previously formed has been supported by a mold support, said thin-wall resilient mold has been filled up with powder and said mold support has been separated from the thin-wall resilient mold, a great increase of accuracy is observed in comparison with the above-described case where the C.I.P. treatment is carried out by the use of the resilient mold having a predetermined thickness and strength (the Japanese Patent Publication Laid Open No. 297402/87).
However, since the resilient mold which is inserted into a ventilative mold support similar in shape to the resilient mold is expanded by the use of a difference in pressures inside and outside the resilient mold and put close to the inner surface of the ventilative mold support, there occurs a phenomenon such that the resilient mold expands, not moving to positions corresponding to due positions of the inner surface of the mold support similar in shape to the resilient mold. When the resilient mold, in which said phenomenon takes place, is subjected to the C.I.P. treatment as it is, there occurs an anisotropic contraction and creases of the resilient mold. The more a desired shape of a compact becomes complicated, the greater this problem is posed.